This invention relates to a novel catalyst composition and to the polymerization of vinyl chloride using that composition. In particular, it relates to a catalyst composition that is a mixture of a vanadyl (V) catalyst and an alkyl aluminum cocatalyst.
Vinyl chloride monomer (VCM) is commercially polymerized to produce polyvinyl chloride (PVC) using peroxide initiators. Because peroxides are unstable and can even be explosive, they must be handled and stored very carefully at a low temperature, which complicates and adds to the cost of the manufacturing process. Moreover, elevated temperatures are usually required in polymerization reactions initiated by peroxides (50-80xc2x0 C.), which could deteriorate the properties of the product. Besides, the free-radical nature of the traditional polymerization process implies that the product properties, like tacticity, molecular weight, and polydispersity, are not influenced in any way by the structure or composition of the radical initiator. Efforts have been made to find non-peroxide catalysts for polymerizing vinyl chloride. For example, U.S. Pat. Nos. 3,775,391, 3,786,032, and 4,129,702 disclose the polymerization of vinyl or vinylidene halides using as an initiator a transition metal compound, such as vanadium oxytrichloride, and an organoaluminum or organozinc compound, such as triethyl aluminum, including a ligand derived from an oxime or a hydroxy ester. These catalysts seem to be highly specific and, like most catalysts, small chemical variations thereof are nonfunctional.
We have discovered a novel organometallic catalyst composition that is useful for polymerizing vinyl chloride monomer alone or with comonomers. The catalyst composition of this invention is a mixture of a vanadyl (V) catalyst and an alkyl aluminum cocatalyst. Unlike peroxide initiators, which require elevated temperatures, the organometallic catalyst composition of this invention can be used not only at high temperatures, but also at ambient or below ambient temperatures. Besides, in the polymerization process using the new catalyst composition, the molecular weight of the PVC can be easily controlled through the catalyst concentration. Also, the catalyst composition is not explosive and the components can be stored at room temperature. The catalyst compositions of this invention can be used in solution, bulk, or vapor phase polymerization of vinyl chloride, the last two processes being the most attractive for economical reasons.
The catalyst composition of this invention is a mixture of a vanadyl (V) catalyst and an alkyl aluminum cocatalyst.
The vanadyl (V) catalyst has the general formula 
where R is a group containing 1 to 14 carbon atoms and X is halogen or OR. Preferably, R is alkyl from C1 to C4 and X is OR because those catalysts are more readily available. Some of the catalysts where R is alkyl, such as vanadyl (V) triisopropoxide, are commercially available. Catalysts that can not be purchased can be easily made by reacting vanadium trihalide oxide with 1, 2, or 3 moles of an alcohol, a phenol, a carboxylic acid, an enolizable ketone, or their salts, a reaction that proceeds readily at room temperature. A vanadyl (V) catalyst can also be prepared by reacting vanadium trihalide oxide with a compound containing an oxirane ring. For example, one mole of vanadium trichloride oxide can be reacted with 1, 2, or 3 moles of ethylene oxide: 
where p is 1, 2, or 3. This reaction proceeds readily at room temperature in 1 to 24 hours. Examples of suitable compounds having an oxirane ring include ethylene oxide, propylene oxide, epichlorohydrin, cyclohexene oxide, and cyclooctene oxide. The preferred vanadyl (V) catalyst is the reaction product of vanadium trichloride oxide and cyclohexene oxide because it is readily available and works well.
The alkyl aluminum cocatalyst has the general formula
(Rxe2x80x2)mAl(Xxe2x80x2)3xe2x88x92m
where each Rxe2x80x2 is independently selected from alkyl from C1to C10, Xxe2x80x2 is halogen, xe2x80x94OR1, xe2x80x94OC(xe2x95x90O)R1, xe2x80x94OAl(Rxe2x80x2)2 or (OAlRxe2x80x2)n, xe2x80x94OAl(Rxe2x80x2)2, R1 is alkyl from C1 to C10, haloalkyl from C1 to C10, alkoxyalkyl where each alkyl group is independently selected from C1 to C10, or aryl from C6 to C14, m is 1 or 2, and n is 1 to 100. Preferably, Rxe2x80x2 is alkyl from C1 to C4, Xxe2x80x2 is halogen, preferably chlorine, R1 is alkyl from C1 to C4, most preferably ethyl, m is 2, and n is 1 to 10, because those compounds are commercially available. Examples of alkyl aluminum compounds that can be used as cocatalysts include diethyl aluminum ethoxide, diethylaluminum chloride, ethylaluminum dichloride, dimethylaluminum chloride, diethylaluminum propionate, and diethylaluminum benzoate. The preferred alkyl aluminum cocatalyst is diethyl aluminum ethoxide because it is readily available and gives good results. Many of the alkyl aluminum compounds included within the scope of the above formula are commercially available. Alkylaluminum cocatalysts where Xxe2x80x2 is OR1 can be easily prepared by reacting a trialkylaluminum compound with an alcohol or a phenol. Similarly, compounds where Xxe2x80x2 is OC(xe2x95x90O)R1 can be easily made by reacting a trialkylaluminum compound with an aliphatic or aromatic carboxylic acid: 
Compounds where Xxe2x80x2 is xe2x80x94OAl(Rxe2x80x2)2 can be prepared by reacting two moles of trialkyl aluminum with one mole of water. For example, if two moles of triethyl aluminum are reacted with one mole of water the following alkyl aluminum compound is believed to be produced, where xe2x80x9cEtxe2x80x9d is ethyl: 
Similarly, compounds where the Xxe2x80x2 group is xe2x80x94(OAlRxe2x80x2)nOAl(Rxe2x80x2)2 can be made by reacting one mole of trialkyl aluminum with one mole of water. For example, if one mole of trialkyl aluminum is reacted with one mole of water, the following reaction occurs: 
where q can be 1 to 100.
The molar ratio of the alkyl aluminum cocatalyst to the vanadyl (V) catalyst in the catalyst composition can vary from about 0.5 to about 15, but is preferably about 1.5 to about 4 as the optimum seems to be at about 2. If less of the alkyl aluminum cocatalyst is used the polymerization is less efficient and more of the alkyl aluminum cocatalyst results in no significant improvement.
The catalyst composition can be prepared by simply mixing together the alkyl aluminum cocatalyst and the vanadyl (V) catalyst in the desired proportion. This can be accomplished before the catalyst composition contacts the monomer or the catalyst and cocatalyst can be added separately to the monomer. For convenience in laboratory experiments, it is sometimes desirable to use a solvent such as hexane, toluene, or tetrahydrofuran (THF). Preferably, if a solvent is desired vinyl chloride monomer itself is used as the solvent so that problems of disposing of a solvent are avoided.
The catalyst composition is preferably supported, particularly if it is used for bulk or vapor phase polymerization. The support can be an insoluble particulate such as alumina, silica, clay, various other inorganic oxides, or polyvinyl chloride; fumed silica or other small particle size silica is preferred. The finely divided support can be mixed with a solution of the catalyst and the solvent removed by filtration, decanting, or evaporation, which deposits the catalyst on the support. The support holds the catalyst composition and prevents it from entering the solution phase. The support also controls the morphology of the polymer so that the polymer is produced as small particles, rather than in large clumps.
The catalyst composition is generally used in an amount of about 0.01 to about 50 mmole vanadium per mole of monomer; less is usually ineffective and more is unnecessary and may be deleterious to the stability of the PVC. Polymerization can occur within a temperature range of xe2x88x9240 to 80xc2x0 C. and within a pressure range of 0 to 200 psig (0 to 1500/KPa). Most of the unsupported catalyst compositions within the scope of this invention are soluble in the monomers.
The catalyst composition of this invention can be used to polymerize vinyl chloride or copolymerize it with other ethylenically unsaturated monomers such as olefins, halogenated olefins, vinyl ethers, or vinyl esters, as well as acrylic esters, nitrites, and amides. The comonomers can be used in amounts not exceeding 50 mol %, most often 35 mol %, of the mixture of comonomers used in the copolymerization. The process according to the invention is very particularly suitable for the homopolymerization of vinyl chloride. The preferred comonomers are vinyl acetate, vinylidene chloride, and methylenenorbornene because they work well and are commercially important.
The vinyl chloride monomer can be polymerized as a slurry, in a solvent, as a pure liquid or liquified gas, or as a vapor, and the polymerization process can be batch or continuous. As the polymer forms, it precipitates and is collected and purified if necessary. It is then compounded with plasticizers and other additives and is processed into various shapes according to methods well known in the art.